The present invention relates generally to an electronic ballast for powering a high-intensity gas discharge lamp such as a high-pressure mercury lamp or a metal halide lamp. More particularly, the present invention relates to circuitry within an electronic ballast for powering a lamp that prevents operation below a resonance frequency.
Electronic ballasts for powering high-intensity discharge lamps such as high-pressure mercury halide lamps or metal halide lamps are conventionally known in the art. Referring to FIG. 5a, a typical example of such a ballast is shown including a voltage step-down circuit 1 that steps down a DC voltage from a direct current power supply (DC) and an inverter circuit 2 including four high-frequency switching elements Q2-Q5 coupled to receive the DC voltage from the step-down circuit and produce a high-frequency AC voltage. An LC resonant circuit 20 is coupled to receive the inverter output and further coupled to the terminals of a discharge lamp (La). An inverter driving circuit 3 outputs a driving signal for controlling the switching elements Q2-Q5. A voltage step-down control circuit 4 regulates the voltage output from the voltage step-down circuit 1.
The voltage step-down circuit 1 of the present example further includes an inductor L, a capacitor C1, a diode D1 and a switching element Q1 such as a synchronous rectifier Q1. The voltage step-down circuit 1 steps down the DC voltage from the DC power supply by switching on and off the switching element Q1 and provides the stepped-down DC voltage across either end of capacitor C1. A current sensor such as a resistor Ra is connected to one end of the DC power supply for detecting a load current flowing through the discharge lamp La. The voltage step-down control circuit 4, which may be a microprocessor, applies a driving signal to the switching element Q1 for controlling the switching rate of the switching element Q1. The voltage step-down control circuit 4 changes a frequency of the driving signal according to the load current detected across the resistor Ra, thereby controlling the voltage output from the voltage step-down circuit 1 to a predetermined voltage level.
The inverter circuit 2 is configured such that a series connected circuit including the switching elements Q2, Q3 is coupled in parallel to a series connected circuit including the switching elements Q4, Q5. The resonant circuit 20 and the discharge lamp La are coupled to a node between the switching elements Q2, Q3 and to another node between the switching elements Q4, Q5. The resonant circuit 20 in this example is an LC circuit including an inductor L2 and a capacitor C2.
A voltage detection circuit 5 that detects the high frequency voltage supplied to the discharge lamp La is coupled to a node between the inductor L2 and the capacitor C2. The voltage detection circuit 5 is configured to include capacitors C3, C4 and diodes D2, D3 that rectify the high frequency voltage, resistors R1-R4 that divide the rectified voltage, and a capacitor C5 that smoothes the rectified voltage. Further, the voltage detection circuit 5 is configured to apply a voltage across either end of the capacitor C5 and to the driving circuit 3.
The driving circuit 3, which may include a microprocessor, supplies a driving signal to each of the switching elements Q2-Q5 so as to alternately switch on and off a first pair of switching elements Q2, Q3 and a second pair of switching elements Q4, Q5, thereby switching the switching elements Q2-Q5 at a high frequency. The driving circuit 3 utilizes frequency sweep control to modulate the driving signal frequency within a predetermined range and to secure proper starting operation even where the inductance or capacitance of components L2, C2 of the resonant circuit 20 vary, or even if the discharge lamp La is at the end of its life and the voltage necessary to start the lamp increases.
Operation of the conventional discharge lamp ballast will be described with reference to FIG. 5b. The entire operation here takes place in a startup mode 22 as will be later distinguished from additional modes in operation of the present invention. First, to obtain the high frequency voltage Vhf necessary to start the discharge lamp La, the driving circuit 3 sweep controls the driving frequency Fdrv to be applied to the switching elements Q2-Q5 to approach the resonant frequency of the resonant circuit 20. In this example, the driving circuit 3 sweep controls the driving frequency Fdrv within a range from a maximum frequency Fmax of 120 kHz to a minimum frequency Fmin of 95 kHz. If the voltage detection circuit 5 detects the high frequency voltage applied to the discharge lamp La as having reached a predetermined voltage threshold such as that necessary to start the lamp, then the driving circuit 3 stops sweep controlling the driving frequency and fixes the driving frequency for a predetermined period of time. In this example, the driving circuit 3 fixes the driving frequency at 105 kHz. The voltage output from the step-down circuit Vout_sd remains constant throughout this operation as shown.
During this predetermined period of time, a high frequency voltage of several tens of kilohertz (kHz) to several hundreds of kHz is supplied to the discharge lamp La, and the discharge lamp La is ignited and lit. If the discharge lamp La is not ignited during this predetermined period of time, the driving circuit 3 changes the driving frequency to an initial frequency from the time of activating the ballast and sweep controls the driving frequency again. After the discharge lamp La is ignited, the driving circuit 3 controls the driving frequency to supply a low frequency voltage of several tens of hertz (Hz) to several hundreds of Hz to the discharge lamp La, and maintains steady-state operation of the lamp.
In accordance with this example, if the driving frequency of the driving circuit 3 is close to the maximum frequency in the predetermined range, the high frequency voltage supplied to the discharge lamp La decreases to be almost identical to a voltage detected immediately after lighting of the discharge lamp La. While it is difficult to promptly determine whether or not the discharge lamp is lit, it will nevertheless be determined within a finite period of time whether the discharge lamp is lit or not. Therefore, the discharge lamp La may be lit in a state in which the high-frequency voltage is low, and if so the driving circuit 3 repeatedly sweep controls the driving frequency from a maximum frequency to a minimum frequency.
In the above-stated case, the high frequency voltage changes according to a resonance characteristic, not at a point where the lamp is unlit, but at a point where the lamp is lit. Due to this, even if the driving circuit 3 sweeps the driving frequency to make the frequency lower, the high frequency voltage is of a reduced magnitude and therefore does not reach the predetermined voltage. As a result, conditions for stopping the driving circuit 3 from sweeping are not met and the driving circuit 3 continues sweeping the driving frequency to below the resonant frequency. No problems occur if the discharge lamp La remains lit. However, when the discharge lamp La is turned off, the resonant circuit 20 operates at a frequency lower than the resonant frequency according to the resonance characteristic present at the time the discharge lamp La was extinguished. The below resonance operation may place excessive and potentially destructive stress on the various switching elements of the ballast circuitry.